The present invention relates to hydraulic brake systems for vehicles such as passenger cars.
Japanese Patent Provisional (KOKAI) Publications Nos. 61-108041 and 61-287852 disclose conventional brake systems. These conventional system have a cutoff valve for cutting off the fluid communication between a brake master cylinder and a hydraulic modulator for anti lock control and traction control, to enable the modulator to control the brake fluid pressure independently of the master cylinder fluid pressure. This cutoff valve has a solenoid which produces a force to close the valve when energized, and a return spring producing a force to open the valve when the solenoid is deenergized. Therefore, if the solenoid is kept energized because of a failure in the electric system, then the cutoff valve continues to be closed, and deteriorates the brake effectiveness and the safety of the vehicle.